1. Field of the Invention
The present invention relates to a signal processing circuit and a physical quantity measuring device using the signal processing circuit, and particularly relates to a magnetic element control device that drives a time-resolution type flux-gate type (hereinafter, referred to as an FG-type) magnetic element, a magnetic element control method, and a magnetic detection device that detects a magnetic field using the magnetic element control method.
2. Description of the Related Art
Generally, FG-type magnetic elements have a high sensitivity of detecting a magnetic field and are capable of a reduction in size, as compared to Hall elements or magneto-resistive elements which are magnetic elements that detect similar magnetism, and thus are used in azimuth detection devices such as portable electronic devices, and the like.
FIG. 11 is a diagram showing a configuration example of a time-resolution FG-type magnetic element (for magnetic proportion type measurement). As shown in FIG. 11, the FG-type magnetic element is configured such that an excitation winding and a detection winding are wound around the circumferential surface of a magnetic substance core which is formed of a high magnetic permeability material. A region around which the excitation winding is wound is driven by an excitation signal as an exciting coil, and a region around which the detection winding is wound outputs a detection signal as a detection coil.
FIG. 12 is a waveform diagram showing a principle of a magnetic proportion system in which magnetism is detected using the time-resolution FG-type magnetic element. PART (a) of FIG. 12 shows an excitation current which is supplied to the exciting coil of the magnetic element, in which the vertical axis thereof represents the current value of the excitation current, and the horizontal axis thereof represents time. PART (b) of FIG. 12 shows a magnetic flux density of a magnetic field which is generated in the magnetic substance core by the exciting coil of the magnetic element, in which the vertical axis thereof represents magnetic flux density, and the horizontal axis thereof represents time. PART (c) of FIG. 12 shows the voltage value of a pulse which is generated by the detection coil of the magnetic element due to an induced electromotive force, in which the horizontal axis thereof represents time.
In FIG. 12, since the exciting coil is driven, a signal of an excitation current Id (hereinafter, referred to as an excitation signal) is applied between the terminals of the exciting coil, as the excitation signal of an alternating current having a constant cycle, that is, as the excitation signal (that is, triangular wave current signal) having a triangular wave shape as shown in PART (b) of FIG. 12.
Thereby, in the time (positive and negative alternating time zone of the excitation current) at which the direction of the excitation current changes, in the case of PART (c) of FIG. 12, the detection coil generates a positive and negative pulse (a pickup signal, that is, a pu signal) due to an induced electromotive force at time t1 and time t2, and a voltage Vp (pickup voltage) of the pulse is set to a detection signal. The detection signal is continuously generated between the terminals of the detection coil as a pulse having voltages of positive and negative polarities, corresponding to the cycle of the triangular wave current signal.
When a stationary magnetic field Hex (see FIG. 11) passing through a cylindrical space in which the excitation winding and the detection winding of the magnetic substance core are created is applied to the magnetic element, a stationary current corresponding to the stationary magnetic field flows in the excitation winding. That is, the above-mentioned stationary current is superimposed, as an offset, on the excitation current Id of the excitation signal which is applied to the excitation winding.
As a result, the driving state of the exciting coil based on the alternating excitation signal changes due to the offset. That is, the time at which the direction of the flow of the excitation current Id changes varies in a case where the stationary magnetic field Hex is applied and a case where the stationary magnetic field Hex is not applied.
In this case, as shown in PART (c) of FIG. 12, when the stationary magnetic field Hex in the same direction as that of a magnetic field generated by the exciting coil is applied (Hex>0), as compared to a case where the stationary magnetic field Hex is not applied (Hex=0), a value of time t1 becomes smaller at a timing at which the direction of the flow of the excitation current Id changes, and a value of time t2 becomes larger (time Tm becomes shorter than T/2). On the other hand, when the stationary magnetic field Hex in an opposite direction to that of a magnetic field generated by the exciting coil is applied (Hex<0), as compared to a case where the stationary magnetic field Hex is not applied, a value of time t1 becomes larger at a timing at which the direction of the flow of the excitation current Id changes, and a value of time t2 becomes smaller (time Tp becomes longer than T/2).
Thereby, a magnetic flux density φ in the magnetic substance core changing with a timing at which the direction of the flow of the excitation current Id changes also varies corresponding to the stationary current which is superimposed on the excitation current Id.
When the direction of a magnetic flux changes, an induced electromotive force is generated in the detection coil in a direction in which a change in magnetic flux is canceled. That is, a detection signal is generated as a pulse of a negative voltage at a timing at which the excitation current Id changes from positive to negative. On the other hand, a detection signal is generated as a pulse of a positive voltage at a timing at which the excitation current Id changes from negative to positive.
Therefore, in the FG-type magnetic element, a timing at which the detection signal is output when the stationary magnetic field Hex is not applied is compared with a timing at which the detection signal is output when the stationary magnetic field Hex is applied, thereby allowing the magnitude of the stationary magnetic field Hex to be measured indirectly. That is, when the stationary magnetic field Hex is applied, a specific stationary current flows to a driving coil. Therefore, a constant offset is superimposed on the excitation signal, and a time interval between pulsed detection signals of a negative voltage and a positive voltage changes.
Therefore, magnetic detection devices using the FG-type magnetic element measure the intensity of the stationary magnetic field Hex applied from the outside by measuring a time interval at which the pulsed detection signals of a negative voltage and a positive voltage are generated (see, for example, Japanese Unexamined Patent Applications, First Publications No. 2008-292325, No. 2007-078423, and No. 2007-078422).
Here, the maximum value of the excitation current Id which is applied to the exciting coil is set to a value for which a magnetic field having more than the saturation magnetic flux density of the magnetic substance core is generated. Thereby, the measurement magnetic field range of the magnetic element is determined from the time of one cycle of the excitation signal, and time change (hereinafter, referred to as excitation efficiency) corresponding to the current value of the stationary current as an offset due to the application of the stationary magnetic field Hex.
That is, a period from time t0 to time t3 is one cycle of the excitation signal, and a cycle width is time T. When the stationary magnetic field Hex is not applied (Hex=0), the time from time t1 at which a detection signal of a negative voltage (hereinafter, referred to as a first detection signal) is output to time t2 at which a detection signal of a positive voltage (hereinafter, referred to as a second detection signal) is detected becomes equal to the half cycle of the excitation signal, and thus is set to time T/2.
In addition, when the stationary magnetic field Hex is applied, a time width (hereinafter, referred to as measurement time width) until the first detection signal is output and then the second detection signal is detected changes with respect to time T/2. As shown in FIG. 11, when the magnetic flux direction of the stationary magnetic field Hex is a solid arrow (Hex>0), the direction is the same as the direction of a magnetic flux which is generated by the exciting coil, and thus time width Tm becomes shorter than time T/2 (T0>Tm). On the other hand, when the magnetic flux direction of the stationary magnetic field Hex is a dashed arrow (Hex<0), the direction is opposite to the direction of a magnetic flux which is generated by the exciting coil, and time width Tp becomes longer than time T/2 (Tp>T0). Here, the relation of T0=T/2 is established.
FIG. 13 is a diagram showing a configuration example of a time-resolution FG-type magnetic element (for magnetic balance type measurement). As shown in FIG. 13, unlike the magnetic element of FIG. 11, the FG-type magnetic element in magnetic balance type measurement is configured such that a feedback (hereinafter, referred to as FB) winding coil is wound around the circumferential surface of the magnetic substance core which is formed of a high magnetic permeability material, in addition to the excitation winding and the detection winding. A region around which the excitation winding is wound is driven by an excitation signal as an exciting coil, a region around which the detection winding is wound outputs a detection signal as a detection coil, and a region around which the feedback winding is wound is driven by a feedback signal as an FB coil.
FIG. 14 is a waveform diagram showing a principle of a magnetic balance type measurement in which magnetism is detected using the time-resolution FG-type magnetic element.
PART (a) of FIG. 14 shows an excitation current which is supplied to the exciting coil of the magnetic element, in which the vertical axis thereof represents a current value of the excitation current, and the horizontal axis thereof represents time. The excitation current is a positive and negative alternating signal bordered by a reference current value of 0 A (zero amperes). PART (b) of FIG. 14 shows an FB signal (that is, a feedback signal) which is a current applied to the FB coil of the magnetic element, in which the vertical axis thereof represents a current value of the FB signal, and the horizontal axis thereof represents time. PART (c) of FIG. 14 shows a voltage value of a pulse which is generated by the detection coil of the magnetic element due to an induced electromotive force, in which the horizontal axis thereof represents time.
As shown in FIG. 14, in the case of magnetic balance type measurement, a magnetic field that cancels out the stationary magnetic field Hex (stationary magnetic field passing through the magnetic substance core) which is applied to the magnetic element is generated by the above FB coil. A stationary magnetic field which is applied to the magnetic element is measured from a current value when the magnetic field that cancels out a stationary magnetic field is generated in the FB coil.
In a magnetic balance system, as a coil that generates a magnetic field that cancels out a stationary magnetic field in the magnetic substance core, the above FB coil is provided in the magnetic element, in addition to the exciting coil and the detection coil.
Hereinafter, in this specification, a method in which a stationary magnetic field in the magnetic substance core is canceled by applying an FB signal and in which a magnetic field is measured is referred to as FB control of an FB coil.
In addition, in the case of magnetic balance type measurement, similarly to the magnetic proportion system described previously, a time interval between pulses generated in the detection coil is measured in the positive and negative alternating time zone of the excitation signal which is applied to the exciting coil. The FB signal is applied to the FB coil so that time from time t1 at which the measured detection signal of a negative voltage is output to time t2 at which the detection signal of a positive voltage is detected becomes equal to T/2.
For example, in PART (c) of FIG. 14, when a time width between time t1 and time t2 is larger than T/2, the stationary magnetic field Hex in a negative direction is applied as shown in PART (a) of FIG. 14, and the curve of the excitation signal changes substantially from curve L0 to curve L2. For this reason, since curve L2 of the excitation signal is returned to a position of curve L0 in which the time width between time t1 and time t2 becomes equal to T/2, the FB signal of the current value of line FB2 in PART (b) of FIG. 14 is applied to the FB coil.
On the other hand, in PART (c) of FIG. 14, when the time width between time t1 and time t2 is smaller than T/2, the stationary magnetic field Hex in a positive direction is applied as shown in PART (a) of FIG. 14, and the curve of the excitation signal changes substantially from curve L0 to curve L1. For this reason, since curve L1 of the excitation signal is returned to the position of curve L0, the FB signal of the current value of line FB1 in PART (b) of FIG. 14 is applied to the FB coil.
The intensity of the stationary magnetic field which is applied to the magnetic element is obtained from the current value of the FB signal applied to the FB coil so that the time width between time t1 and time t2 becomes equal to T/2.
Next, FIG. 15 is a block diagram showing a configuration example of a magnetic detection device using a magnetic element control device in FB control of an FB coil. In FIG. 15, a magnetic element 300 is constituted by a detection coil, an exciting coil, and an FB coil.
A magnetic element control device 200 is constituted by a magnetic element control unit 201, a clock signal generation unit 202, and a clock signal adjustment unit 203.
The clock signal generation unit 202 generates a clock of cycle T, and outputs the generated clock to the clock signal adjustment unit 203.
The clock signal adjustment unit 203 adjusts the signal level of the clock to be supplied, and outputs the adjusted clock to the magnetic element control unit 201.
The magnetic element control unit 201 includes a detection signal amplification unit 2012, a detection signal comparison unit 2013, a feedback signal adjustment unit 2014, a feedback signal conversion unit 2015, a data signal conversion unit 2016, an excitation signal adjustment unit 2017, and an excitation signal generation unit 2018.
The excitation signal generation unit 2018 generates a triangular wave as the excitation signal shown in PART (a) of FIG. 14 from the clock which is supplied from the clock signal adjustment unit 203.
The excitation signal adjustment unit 2017 adjusts the voltage level of the excitation signal which is supplied from the excitation signal generation unit 2018, and supplies the adjusted voltage level, as the excitation signal, to the exciting coil.
The exciting coil generates a magnetic field corresponding to the triangular wave within the magnetic substance core of the magnetic element 300.
The detection coil generates a pulse at the positive and negative alternating time zone of the excitation signal in the magnetic substance core.
The detection signal amplification unit 2012 amplifies the voltage level of the pulse which is supplied from the detection coil, and outputs the amplified voltage level, as the detection signal, to the detection signal comparison unit 2013.
The detection signal comparison unit 2013 obtains a difference between T/2 and the time width of the pulse (detection signal) between time t1 and time t2, and outputs the difference to the feedback signal conversion unit 2015.
The feedback signal conversion unit 2015 obtains the current value of the FB signal, supplied to the FB coil, from the supplied difference.
Here, the feedback signal conversion unit 2015 reads out the current value corresponding to the difference from an FB current value table which is previously written and stored in an internal storage unit, and obtains the current value of the FB signal.
The FB current value table is a table indicating the correspondence of the above difference to a current value (digital value) for cancel a stationary magnetic field in the magnetic substance core.
The feedback signal adjustment unit 2014 performs D/A (Digital/Analog) conversion on the current value of the FB signal which is supplied from the feedback signal conversion unit 2015, and outputs the generated current as the FB signal to the FB coil. In addition, the feedback signal adjustment unit 2014 outputs the current value of the FB signal, supplied from the feedback signal conversion unit 2015, to the data signal conversion unit 2016.
The feedback signal adjustment unit 2014 obtains the intensity of the stationary magnetic field canceled in the magnetic substance core, that is, the intensity of the stationary magnetic field Hex applied to the magnetic element 300, from the current value of the FB signal to be supplied. Here, the feedback signal adjustment unit 2014 reads out the magnetic field intensity corresponding to the current value of the FB signal, from a current value magnetic field table which is previously written and stored in an internal storage unit, and obtains the intensity of the magnetic field which is applied to the magnetic element 300. The current value magnetic field table is a table indicating the correspondence of the above current value of the FB signal to the intensity of the applied stationary magnetic field Hex.
When magnetism of the magnetic proportion system is detected using the above-mentioned time-resolution FG-type magnetic element, a measurable magnetic field range is determined by the intensity of the excitation signal and the amount of magnetic field generated per current applied to the coil (hereinafter, referred to as excitation efficiency) which is caused by the material and structure of the magnetic substance core of the magnetic element 300.
When magnetism of the magnetic balance type is detected using the time-resolution FG-type magnetic element, a magnetic field within the magnetic substance core is maintained in an equilibrium state so that the detection signal is output at a constant time interval (T/2) regardless of the stationary magnetic field Hex which is applied to the magnetic element 300. For this reason, a restriction can be performed by the power supply voltage of the entire magnetic element 300, that is, the measurement of the magnetic field can be performed in a range in which the current value of the FB signal is capable of being supplied.
In addition, when magnetism of the magnetic proportion system is detected using the time-resolution FG-type magnetic element, a time interval at which the detection signal is output changes depending on the magnetic field, and thus the linearity of magnetic sensitivity is reflected directly to the characteristics of the magnetic element 300.
On the other hand, when magnetism of the magnetic balance system is detected using the time-resolution FG-type magnetic element, the magnetic field dependency of excitation efficiency is small as the characteristics of the magnetic element, and thus the waveform of the detection signal and the stationarity of a time interval at which the detection signal is generated have a tendency to be maintained.
For this reason, when a measuring object is applied to the magnetic element that measures a magnetic field which is generated by a current of approximately several hundred A (amperes) in the entire measurement current range in a state where linearity is maintained, magnetism detection in the magnetic balance system has been mainly used so far, as compared to the magnetic proportion system.
When magnetism is detected by the magnetic proportion system using the above-mentioned time-resolution FG-type magnetic element, as previously stated, the measurable magnetic field range is restricted by the excitation signal and the excitation efficiency of the magnetic element 300.
For this reason, when the magnetic element which is the magnetic proportion system is applied as a current sensor having a maximum measurement current of approximately several hundred A, the measurement range of a magnetic field capable of obtaining high-accuracy output linearity is restricted due to the restriction of an allowable maximum current value or a power supply voltage used to drive the magnetic element, in addition to the dependency of the output linearity of a single magnetic element on the intensity of a magnetic field.
In addition, when the waveform of the detection signal generated by the detection coil changes depending on the intensity of the stationary magnetic field Hex and the temperature of the magnetic substance core, there is a correlation between the time differential value of a rise in the waveform of the detection signal and the output variation of the detection signal. For this reason, the time variation value of the output of the detection signal changes depending on the intensity of the magnetic field. Thereby, in the measurement of the intensity of a magnetic field, particularly, as the intensity of the magnetic field increases, the time variation value increases, and a magnetic field is not able to be detected with a high degree of accuracy.
On the other hand, when magnetism is detected by the magnetic balance system using the time-resolution FG-type magnetic element, the FB signal is generally performed by current control in FB control of an FB coil.
As previously stated, even when there is a proportional relation between the current value in an FB control signal and the intensity of a magnetic field generated by the current value, and the resistance of the FB coil changes corresponding to a temperature due to the difference in the current value of the FB signal, the current value of the FB signal is controlled at a constant current. For this reason, in the magnetic field having a high intensity in which the current value of the FB signal increases, it is also possible to maintain the sensitivity linearity of the magnetic element.
In addition, even when each excitation efficiency of the exciting coil and the FB coil changes with the individual deviation of the characteristics of the magnetic element, the convergence state of magnetic field equilibrium between the magnetic field generated by the FB signal and the stationary magnetic field Hex applied to the magnetic element 300 is restricted by the characteristics of the control circuit that outputs the FB signal, and a residual error (error) in convergence is not generated.
Further, when the ratio of the excitation efficiency of the exciting coil to the excitation efficiency of the FB coil is held constant, the magnetic sensitivity ratio of the exciting coil to the FB coil does not change, and thus the convergence time until the magnetic field based on the FB signal and the stationary magnetic field reach magnetic field equilibrium also does not change.
Therefore, when the exciting coil and the FB coil in the magnetic element are simultaneously formed by a semiconductor process or the like, a coil resistance ratio is maintained even in a case where each resistance of the exciting coil and the FB coil changes. Thus, a residual error in an equilibrium state which is an index of the convergence of magnetic field equilibrium does not occur, and the time to reach the equilibrium state does not change.
However, when magnetism is detected by the magnetic balance system using the time-resolution FG-type magnetic element, the magnetic element is provided with the FB coil, and thus a reduction in the size of the magnetic element is restricted. In addition, when the FB signal to drive the FB coil controls the intensity of a magnetic field generated by the FB coil based on the current value, the current value corresponding to the intensity of a magnetic field is required to be determined by controlling constant current. For this reason, a voltage-to-current conversion circuit that controls a constant current has to be mounted. Therefore, the circuit size of a control unit that controls a current which is applied to the FB coil becomes larger, and the consumption current also increases.
In addition, an internal reference potential when a constant current in the voltage-to-current conversion circuit is generated fluctuates temporally in association with an increase in the current value of the FB signal, and thus becomes unstable. Therefore, the current value of a constant current to be output fluctuates.
The present invention is contrived in view of such circumstances, and an object thereof is to provide a magnetic element control device, a magnetic element control method and a magnetic detection device in which magnetism of a magnetic balance system employing a time-resolution FG-type magnetic element is detected using a magnetic element constituted by only an exciting coil and a detection coil, and which is provided with an offset adjustment function (same as that of) for a voltage-to-current conversion circuit that provides an excitation signal for current control of an FB coil.